The present invention relates to an improvement of alkaline storage batteries such as nickel-cadmium storage battery, nickel-metal hydride storage battery and the like which have a nickel electrode.
In recent years, alkaline storage batteries, especially small-size sealed type batteries, have been used extensively as a main power source for various kinds of portable equipment including communication apparatuses, business machines, home electric appliances, sundry goods, etc. because of their balanced advantages over other battery systems in charge and discharge characteristics, cycle life, safety and reliability. Also, because they have very excellent charge and discharge characteristics and reliability, alkaline storage batteries attract attention as a large power source, a mobile main power source for electric vehicles, for example.
The storage battery system industrially representing those alkaline storage batteries is the nickel-cadmium storage battery which has a long history. Recently, the nickel-metal hydride storage battery in which a metal hydride is used in place of the cadmium negative electrode of the storage battery has been industrialized and is fast growing in market share because of the strength of its high energy density.
As has been attempted in the past, to improve such energy density and reliability, extremely important is: (1) to make lighter, thinner, shorter and smaller the supporting member and additives in the electrode, separator, battery case and lid and others so as to allow the packing of a large amount of active materials of the positive and negative electrodes in a certain volume; (2) to develop a new active material that displays a high energy density under various application conditions; and (3) to improve additives, conductive materials, etc. to raise the utilization of the active material. So, the recent technical trends of these are hereinafter described.
In the positive electrode in the alkaline storage battery, the main active material is a nickel oxide (NiOOH) which has been long used and not changed, but the traditional sintered plaque as a substrate of the electrode has begun to be replaced by the network substrate with a higher porosity such as, for example, a foamed nickel substrate. As a result, an electrode with a large amount of active material powder filled in the foamed nickel substrate has been industrialized which has remarkably improved the energy density of the positive nickel electrode (Japanese Published Patent Sho 62-54235, U.S. Pat. No. 4,251,603). The industrialization of an electrode with a nickel felt as a substrate is also reported which has the same characteristics as the foamed nickel substrate.
The common advantage in application of these high-porosity substrates is that a simple manufacturing process can be used; because the pore size can be made large unlike the conventional micro-porosity sintered plaque, nickel oxide in the form of paste can be directly filled into the substrate. But, a powder of large particle size is filled into the substrate which has a much large pore size than that of the sintered plaque. This causes a decrease in electric conduction between the active material powders and the substrate, or between the active material powders, which in addition to the decrease in electronic conductivity of the whole electrode plate supporting the active material powder, has a marked influence, and induces decreased utilization of the active material. So, the conductivity has been compensated by adding Co, its oxide or nickel etc. to the active material powder, and the still insufficient conductivity has been made up by incorporating metallic elements other than Ni, such as Co, etc. into the nickel oxide to form a solid solution.
It was found that the incorporation into the nickel oxide of other elements in a solid solution state is markedly effective in improving charging efficiency and that the incorporation of two elements Co and Cd especially has a remarkable effect, particularly. Then, Zn, which resembles Cd in characteristics, has drawn attention and is used as a substitute element for Cd, and also a solid solution having three elements Co, Zn and Ba incorporated therein is proposed. Such incorporation into nickel oxide of other elements in a solid solution state for the purpose of raising the efficiency of discharge characteristics is an old-established technology as far as the non-sintered type electrode is concerned, and improved examples are cited which are achieved by using a solid solution nickel oxide incorporated with one element selected from among Mg, Ca, Ba, Ti, Zr, Mn, Co, Fe, Cu, Sc, Y, etc.
The incorporation into nickel oxide of such elements as Co, Cd, Zn, etc. in a solid solution state is effective in not only improving charging efficiency but also inhibiting the formation of a highly oxidized compound, that is, nickel oxyhydroxide of the .gamma. type. Therefore, the incorporation of the above-mentioned elements, which keep down the volume swelling of nickel oxide, is an effective way to lengthen the cycle life if it is used in brittle, foamed metal-type electrode, etc.
In parallel with the improvement from the material side of active material, the shape of the active material powder has been improved into a sphere which is suitable for high density packing and has begun to be used in storage batteries for practical use.
The method of adding the above-mentioned Co and other oxides has been further improved. The following methods have been proposed; a way of forming a covering layer of Co(OH).sub.2 on the surface of an active material powder and a way of forming a powder layer of Co oxide. Any of those aims at achieving a high-efficiency utilization and improved productivity of the active material by realizing an efficient method of adding the conductive agent.
As a result of such a technological progress, the charging efficiency of the active material packed in a density far larger than before could be raised to the same level as that of an excellent sintered-type electrode, and the energy density of the positive electrode has drastically improved. A positive nickel electrode with an energy density of 600 mAh/cm.sup.3 or so is now put to practical use.
In the negative electrode, meanwhile, the energy density has greatly increased as a high capacity density metal hydride (AB.sub.5 type) has begun to be used in storage batteries in place of the conventional cadmium negative electrode, and a negative electrode has now been commercialized that has twice or more as large an energy density per unit volume as the positive electrode. Thickness reduction of the separator and component parts related to the battery case progressed rapidly, with the steady increase in the energy density of the storage battery.
However, as indicated above, the call has been increasingly growing in recent years for improvement of energy density as the power source for portable equipment. To achieve further improvement of storage battery energy density in answer to this call, there is a strong demand for still higher energy density and efficiency especially in the positive electrode in association with the development of technology for the negative electrode with high energy density.
Viewed from the aspect of recent uses, in addition to request for long cycle life and safety, there is a stronger call for high energy density over a wider temperature range than before, especially at high temperatures between 40 and 60.degree. C. or so with diversification of application conditions of portable electronic equipment where the storage battery is used as a power source. This is also the case with a large, mobile power source of which the size and weight reduction is called for under vigorous operating and environmental conditions.
As a positive electrode active material for alkaline storage batteries in industrial application, now used is a material based on nickel oxide (Ni(OH).sub.2). The charging and discharging reaction, as shown hereinafter, is said to be mainly a one-electron reaction of Ni between bivalent and trivalent among the crystals of .beta.-Ni (OH).sub.2 phase (.beta. (bivalent) phase) and .beta.-NiOOH phase (.beta. (trivalent) phase). ##STR1##
In actual storage batteries, however, it seems that the reaction occurs between about 2.2 valent and about 3.2 valent (in this case, it is often referred to as a reaction of .beta.-Ni(OH).sub.2 phase and .beta.-NiOOH phase). Anyway, it is a reaction equivalent to approximately one electron. With regard to .beta.-NiOOH in charged state, if the charging is conducted in a low temperature environment or charging is conducted for a long period of time, or usual overcharging is repeated, a part thereof is oxidized to form .gamma.-NiOOH having a higher Ni oxidation state. When it is oxidized to form .gamma.-NiOOH, the volume swells, and the electrode becomes liable to swell. .gamma.-NiOOH is an electrochemically inert material. On this ground, there are such drawbacks as decreased capacity and lowered battery voltage during discharge with the rise of overvoltage due to .gamma.-NiOOH formation. So, various measures to suppress the formation of .gamma.-NiOOH have been taken in the past.
It is noted that .gamma.-NiOOH is represented by the formula A.sub.x H.sub.y NiO.sub.2.nH.sub.2 O in which an alkali metal A is intercalated between the layers composed of Ni and O, thereby to balance a charge between A, H, Ni and O. And the mean valence of Ni is 3.3 to 3.8, and, in the concrete, such values as 3.67 and 3.75 are reported (J. Power Sources 8, p. 229 (1982)). .gamma.-NiOOH.sub.2 is known as a higher oxide compound that indicates a non-stoichiometric valence.
Contrary to that, a number of studies have been made from a viewpoint that to achieve a still higher energy density using a material based on a nickel oxide as the active material for secondary batteries, it is necessary to make a good use of this .gamma.-NiOOH.
From the active material side, for example, there are reported methods of making, from the start, a nickel based hydroxide having an interlayer distance of about 8 angstroms (which resembles the.gamma. type having an interlayer distance of about 7 angstroms but is in a discharged state and often called .alpha.-Ni(OH).sub.2 phase (hereinafter referred to as .alpha. phase) by incorporating a higher oxidized compound forming element such as Mn, Fe, etc. into a nickel oxide so as to form positively charged metal oxide layers, and then incorporating anions between the metal oxide layers for overall charge balance (Solid State Ionics 32/33, p. 104 (1989), J. Power Sources, 35, p.294 (1991), U.S. Pat. No. 5,348,822 etc.).
Though such oxides are readily charged into a higher oxidized nickel oxide thereby increasing the discharging reaction valence, the material density of the material itself falls extremely, because an a phase having a wide interlayer distance is present. Also, if an .alpha. phase recognizable on an X-ray diffraction is present, it will hinder the crystal particle from growing in a high density in the precipitation reaction. Thus, there arises a problem that the tap density of the oxide powder falls greatly. Since the tap density has a positive correlation with the filling density in making an electrode, it will be very difficult to fill an active material in a high density if the tap density drops sharply. From the view of achieving a high energy density, such decrease in tap density is quite a serious problem. This decreases the energy density of the electrode, and the practicability of the electrode is very low.
In the meantime, considerable researches have been conducted from the aspect of electrolyte concentration. In the past, an electrolyte having a specific gravity of 1.2 to 1.3 or so, that is, one which includes an aqueous solution of KOH as a main component containing KOH or NaOH corresponding to 6 to 7N has been generally used in many cases. More specifically, in the nickel-metal hydride storage battery, for example, a 5N-KOH+1N-LiOH aqueous solution (Phillips J. Res. 39 Suppl. No. 1, p. 1), 7N-KOH+0.5N-LiOH (Power Source 12, p. 203 (1988)), etc. and in the nickel-cadmium storage battery which forms H.sub.2 O in charging, one with 15 to 45 g/l of LiOH.H.sub.2 O added to an aqueous solution of KOH having a little higher specific gravity than that, that is, 1.27 to 1.35 (Japanese Laid-Open Patent Sho 60-124368) have been used as an electrolyte.
In this connection, the idea has long been proposed that .gamma.-NiOOH is made to be formed efficiently thereby raising the utilization of the active material, by raising the concentration of the electrolyte. In the nickel-cadmium storage battery, for example, an idea is presented that .gamma.-NiOOH is made to be efficiently formed by setting the concentration of an electrolyte in the formation to 10N so as to raise the utilization of the active material (Japanese Laid-Open Patent Hei 5-144467). More ideas are disclosed, including one in which an electrolyte with the concentration of KOH and LiOH adjusted to 35 to 39 weight percent is used in the nickel-metal hydride storage battery having a positive electrode made up of 85 to 98 weight percent of Ni(OH).sub.2, 1 to 7 weight percent of CoO and 1 to 7 weight percent of ZnO (Japanese Laid-Open Patent Hei 6-283195) and another in which the specific gravity of an electrolyte consisting of KOH, NaOH and LiOH is set to 1.31 to 1.4 (Japanese Laid-Open Patent. Hei 6-45002).
However, the active materials used in those ideas were Ni(OH).sub.2 which form an inert .gamma.-NiOOH that does not discharge until a battery voltage of 1.0 V or one that is so improved as to control .gamma.-NiOOH as mentioned earlier. Therefore, the formation of .gamma.-NiOOH which discharges under the voltage of the battery in service is low, and they have not been an effective way to raise the utilization of the active material.
.gamma.-NiOOH was considered to be inert. But it has been found that with some elements incorporated in a solid solution state, even with a nickel hydroxide of the .beta. (bivalent) phase suitable for high density filling in an uncharged state, a .gamma.-NiOOH phase (hereinafter called .gamma. phase) is formed during charging, and this is ready to discharge under the voltage range of the commonly used storage battery and returns to nickel hydroxide of the .beta. (bivalent) phase. Such an active material has a high density which is advantageous in making an electrode, and in discharging and charging, furthermore, the higher oxidized compound .gamma. phase is utilized. Therefore, the use of such an active material is extremely effective in raising the energy density. In achieving a high efficiency under various service conditions as required recently, it is considered to be effective to incorporate nickel oxide with an element in a solid solution state that raises the charging efficiency of the positive electrode at a high temperature. In the incorporation into nickel oxide of an element for raising the charging efficiency at a high temperature, too, measures are taken by creating a solid solution on the surface layer of the nickel oxide so as to minimize the decrease of the content of Ni, the main component responsible for oxidation-reduction reaction in the active material, thereby raising the energy density of the positive electrode. At any rate, to realize an efficient use of nickel oxide of the .gamma. type is the main way to achieve a high energy density.
However, the electrode using such a newly developed material was found to have a problem that under the conditions where the quantity of the electrolyte is limited as in the sealed battery, the utilization of the active material is some 10 to 25 percent lower than that attained when the electrolyte is used enough and abundantly for the capacity. This decrease in utilization of the active material has been a serious obstacle in providing an alkaline storage battery with a high energy density and high efficiency under the conditions where the quantity of the electrolyte is limited.